High-speed paper printers are, of course, well known. To increase paper handling capability and afford flexibility in paper handling, printers have been provided with multiple paper paths for feeding different types of paper through the printer for different purposes without the necessity of manually removing one type of paper from the printer in order to feed a different type of paper. A printer of this type is described and illustrated in U.S. Pat. No. 5,051,010, of common assignee herewith. In that printer, a plurality of paper paths are provided through the printer, with at least two of the paper paths having a tractor with sprockets for engaging marginal holes along the paper to drive the paper through the printer. Another paper path is provided for printing cut paper. Thus, it becomes necessary in serial dot matrix impact printers of that type with co-resident paper paths to park the paper not currently being printed while printing paper from another path. Clutches are typically used to control the movement of the paper along the various paper paths. Prior printers, however, use clutches that can lose synchronization between the input and output elements when they are disengaged. Upon reengagement, the input rotational element cannot be guaranteed to be at the same rotational angle with respect to the output rotational element prior to disengagement. Maintaining synchronization is of particular importance in maintaining vertical paper position accuracy in the printer, especially during repeated disengagement/reengagement cycles as is the case in multiple paper path printers.
As a specific example, paper may be parked along one of the paper feed paths with the clutch driving the tractor for that paper path disengaged. The clutch for another paper path remains engaged and paper is fed therealong through the printer. However, when the clutch for driving the paper along the one paper path is disengaged, mechanical synchronization between the software operating the printer and the paper in the one path is lost. Similarly, when the clutch for the other paper path is disengaged, synchronization between the software and the paper being driven through the other paper path is lost. In other words, when the clutch in any paper path is disengaged, the link between the paper in that path and the software controlling printer operations is broken. The software does not know the location of the paper parked along the one path when the printer returns to feed paper along that one path. Absent synchronization, it is not infrequent that after multiple engagements and disengagements of the clutches, the printing on the paper will be offset from the desired printing locations on the paper and may possibly print close to or along perforation lines where the paper is to be torn into discrete sheets.
Additionally, where automatic paper shearers are provided on printers of this type, these shearers are oftentimes misregistered with the desired cut lines along the paper, resulting in discrete paper sheets cut too close to the print or even through the print. That is, vertical paper misregistration can be aggravated using previous clutches in printers equipped with paper shearing features where paper from each path is repeatedly sheared and parked. Each time paper is sheared, a new edge is formed. Attempts to sense the paper, for example, optically or by a microswitch, often fail because inaccuracy in the sheared edge will cause the paper to be sensed at a different point. Each time a new paper edge is sensed, an error can occur. These errors are cumulative and eventually can cause gross vertical paper misregistration.